A leaky tube in a tube-type heat exchanger, such as a steam generator, feedwater heater, condenser, or the like, permits contamination of the fluid on either the tube side or the shell side of the heat exchanger, permits loss of pressure, and reduces the efficiency of the heat exchanger. A number of techniques and different types of tube plugs are available to plug defective heat exchanger tubes, but these have been deficient in one or more respects. For example, some are ineffective to produce a leak-tight seal where there are irregularities in the tube surface on or immediately adjacent to the open end of the tube which is to be plugged, such as might be due to corrosion, or when there has been weld build-up or a deposit of weld splatter on the tube when it was initially installed. Other tube plugging devices may damage adjacent tubes, or the tube sheet ligaments between the tubes, by requiring or permitting excessive pressure to be applied as the plug is installed, sometimes causing more leaks than they correct. Still others are difficult to install, especially while working in the tight quarters and other adverse environmental circumstances of a boiler or heat exchanger, such that the tube plugging operation cannot be conducted rapidly and effectively.
Probably the best of the prior art tube plugging devices utilizes a deformable cylindrical bushing or sleeve member which is pressed into contact with the interior tube surface immediately adjacent to the tube end by a relatively hard plug member, which expands the sleeve when forced into it, the plug member then being left within the sleeve and becoming a part of the plug. Usually, the deformable sleeve or bushing member has a tapered interior surface against which a correspondingly tapered outer surface of the plug member presses as the plug member is forced in axial direction against the radially expandable sleeve member. Examples of such devices are disclosed in U.S. Pat. Nos. 3,156,373, 3,525,365, 3,983,904 and 4,091,841. A similar form of sleeve-type plug is the "Elliot" plug, which utilizes a tapered pin forced into a tapered sleeve having an external peripheral shoulder thereon which positions the sleeve against the open end of the tube. The "Elliot" plug effects a seal substantially within the plane of the tube end. Similar devices, but in which only the plug members are tapered, are shown in U.S. Pat. Nos. 4,114,654 and 4,170,247.
The use of such a deformable sleeve as part of the plug represents an improvement over a tapered pin plug, which does not employ such a sleeve and which also continues to be used. However, when either the plain pin plug or any of these prior sleeve-type plugs is used, an acceptable seal is not always provided where the pin or the sleeve is disposed at or adjacent to a tube end having the aforementioned irregularities in its surface, due to the effects of corrosion, weld build-up or splatter, etc. The irregular surface provides interstices between the interior end surface of the tube and the plug when the latter is pressed therein. Thus, the pitting effects of corrosion, and excess weld material on and adjacent to the mouth of the tube must usually be removed by drilling, filing, or reaming in advance of placement of the plug, to permit the plug to seal within the tube end.
Another significant disadvantage of any plug which is placed and driven home at the end of a tube is that any weld spillover at the mouth of the tube will also cause uneveness of driving pressures, with possible resulting distortion of the tube sheet ligaments as the pin is hammered or otherwise tightened. This frequently results in leaks, not only in the seal being formed, but in the adjacent tube to tube sheet joints. Moreover, such plugs are difficult to tighten within the funnel-shaped tube end, and sometimes drop out.
Explosive tube plugs such as shown in U.S. Pat. No. 3,555,656 are also known, these comprising a closed end, hollow metal nipple containing a controlled amount of explosive. Positioned within the mouth of the tube, its detonation swells the nipple, pressing it radially outward against the tube interior at a location spaced inwardly from the tube end. However, such plugs are dangerous in that handling of "live" explosives is required, and exact control of the pressure exerted on the tube is not attainable. Over-expansion of the nipple by an enlarged explosive charge causes distortion of the adjacent tube sheet ligaments, possibly resulting in leaks at adjacent tube joints.
By contrast, the present invention provides a plugging technique which is effective to develop a leak-tight seal without danger of damage to adjacent tubes or tube to tube sheet joints, and in which the plug can be effectively installed in a rapid manner working within the hostile environment of a heat exchanger where temperature and space limitations create poor working conditions. Moreover, the present invention provides a means whereby a pug may be installed at any desired location along the length of the heat exchanger tube, although it is preferable that the tube be plugged within the length of the tube sheet because the tube can then be "pinched" between the expanded plug and the tube sheet to ensure an effective seal.
Further, using the present invention, weld build-up at the mouth of the tube as well as deposited weld splatter material on the inside surface of the tube at or near its mouth can usually be ignored, and therefore need not be removed prior to plugging the tube. Another advantage of the plugging device to be described is that it is possible to control, and thereby to limit the amount of expansion of the plug sleeve, thus eliminating the possibility for damage to adjacent tube sheet ligaments or to adjacent tubes. Further, because of the controlled amount of expansion of the plug sleeve member as is attained by the present invention, it is generally not necessary to consider the relative coefficients of expansion of the plug, the tube, and the tube sheet as is ordinarily the case when installing previously known plugs, because thermal expansion changes will be small as compared with the mechanical expansion of the plug.
Another advantage of the present invention is that a sleeve and plug of a given size can be used to plug any conduit of the same nominal size because, upon installation, the plug will accommodate any normal manufacturing tolerance variation in the interior diameter of the tube.